The present invention relates to fermentation process, and provides a method and apparatus for supporting a fermentation process with the use of substantially pure oxygen.
Fermentation is a process whereby a chemical change is induced by a living organism, or by an enzyme produced by an organism. Normally, such organisms are unicellular plants such as yeast, molds, or fungi. The fermentation reactions can be anaerobic, i.e. with no oxygen added, or aerobic, i.e. oxygen-dependent.
Whether the fermentation process is aerobic or anaerobic depends on the particular microorganisms used for the process, and not necessarily on the final product. The choice of whether to use an aerobic or an anaerobic process often depends on practical considerations. For example, citric acid can be made in either way, but for manufacturing in commercial quantities, it is preferred to use an aerobic method, due to economic factors such as substrate, yield, etc.
Optimum cell growth and product formation depend on the design of the fermentation medium. Care must be taken to provide a sufficient amount of air, required trace elements, and the specific nutritional requirements of the cell. Microorganisms will consume glucose as an energy source in preference to any other carbon compounds. The amount of sugar to be charged into a medium is calculated from the maximum population that a fermenter will support aerobically. Also, assimilable nitrogen must be available. Most cells will use ammonia as readily as amino acids. Consequently, ammonium salts are frequently included in the prepared medium.
The yield of the product being made by the fermentation process is ultimately dependent on cell growth of the microorganisms used. Based on the shape of the growth curve for aerobic fermentation, it turns out that the rate of growth is most critical at the beginning of the fermentation cycle, and it is that portion of the cycle that should be emphasized to maximize yield.
For an aerobic fermentation process, the rate of cell growth is, in turn, dependent on the rate at which oxygen is absorbed into the system. It is an aim of the present invention to enhance such absorption of oxygen. Many valuable chemicals, food, beverages, pharmaceuticals, and farm products are produced by aerobic fermentation. To meet an increasing demand for the final product, the productivity of the process is boosted by a high-strength broth requiring an enhanced oxygen supply. Oxygen demand is highest during the phase of the fermentation process in which the cells are growing exponentially. In this phase, extensive primary metabolism creates a very high oxygen demand, which must be met in order to stimulate cell growth. High viscosity in this phase inhibits oxygen transfer, resulting in oxygen-starved conditions and lower yield.
There are two major kinds of fermentation systems. A mechanically-agitated fermenter comprises a vessel having a mechanical device for agitating the contents of the vessel. Typically, the mechanical device includes a shaft having multiple impeller blades. An air-lifted fermenter does not use a mechanical agitator, but instead relies only on bubbles of air, passing through the contents of the vessel, both to maximize oxygen transfer and to agitate the contents. A product of the fermentation process is carbon dioxide. Unless vented to the outside, the carbon dioxide forms carbonic acid, which will kill the microorganisms used in the fermentation process. Thus, a practical commercial fermentation process must include means for removing carbon dioxide.
Air contains about 21% oxygen, with the balance being about 78% nitrogen and about 1% other gases. When air is used as the sole source of oxygen in a fermentation process, movement of the air can be used to remove the carbon dioxide. Due to the low concentration of oxygen in ordinary air, most of the oxygen available from the air remains undissolved and vents from the fermenter to the atmosphere. An aerobic fermentation process works with dissolved oxygen; any oxygen that is not dissolved will not affect the process. The venting of undissolved air makes it difficult to obtain even the minimal desired level of dissolved oxygen, required to sustain the microorganism growth needed to achieve desired production levels.
A common solution to the above problem with air-based fermentation systems is to increase the air flow. But this technique is helpful only when the oxygen demand is moderate. If the reaction has a high rate of oxygen uptake, an increased flow of air tends to flood the impellers in a mechanically-agitated fermenter. In an air-lifted fermenter, an increased flow of air can fluidize the entire contents of the vessel, and can blow the contents out of the fermenter.
Installing larger agitators and motors may improve the oxygen transfer rate in the fermentation vessel, but doing so is expensive. Even if the capital expenditure is of no concern, large agitators and more powerful motors can provide only incremental improvements in the oxygen transfer rate.
Another possible solution to the problem of increasing the amount of oxygen delivered to a fermentation process is to use oxygen-enriched air. The enriched air can be created by adding pure oxygen to a stream of ordinary air before it enters the vessel. Due to the fire hazard associated with the use of oxygen, care must be taken to be sure that oxygen does not flow back into the air conduit. Also, care must be taken to prevent oil from leaking from the air compressor, so as to prevent such oil from coming into contact with the oxygen.
Because enriched air is distributed to the contents of the vessel using the same sparger that would be used with ordinary air, the dissolution efficiency of enriched air is just as poor as that of ordinary air. Moreover, the use of pure oxygen in addition to ordinary air adds to the cost of the system, because one must manage two separate supply sources. Thus, the use of enriched air in a fermentation system is only marginally economical, and of only limited benefit.
The present invention solves the above problems, by providing a system and method in which substantially pure oxygen is safely injected into a fermentation vessel. The present invention provides an improved fermentation process and apparatus, having substantially improved efficiency, and in which the cost of operation is greatly reduced.